1. Field of the Invention
The present invention relates to a current supply circuit and, more particularly, to a current supply circuit for supplying a current according to display luminance instructed to a current-driven light emitting element, and an electroluminescence (EL) display apparatus having the same.
2. Description of the Background Art
In recent years, in the field of a flat panel display in which a liquid crystal display is typically used, attention is being paid to an organic EL display. The organic EL display has advantages of higher contrast ratio, higher response, and wider angle of visibility as compared with a liquid crystal display. In the organic EL display, an organic EL element as a current-driven light emitting element is arranged for each pixel. A representative example of the organic EL element is an organic light emitting diode.
Particularly, in recent years, among such organic EL displays, from the viewpoints of higher definition of an image and lower power consumption, attention is being paid to a low-temperature polysilicon TFT display using, as a drive device of an organic light emitting diode, a thin film transistor (TFT) using low-temperature polysilicon. However, manufacture variation of transistor characteristics such as mobility and threshold voltage of the low-temperature polysilicon TFT tends to be relatively large as compared with that of a conventional TFT.
In such a background, a problem of non-uniformity of a display luminance characteristic of pixels, that is, variation in display luminance has been pointed out as one of the problems of the organic EL display. As a configuration for solving the problem, a configuration of a so-called “current-programmed pixel circuit” is disclosed in “Pixel-Driving Methods for Large-Sized Poly-Si AM-OLED Displays”, Akira Yumoto et al., Asia Display/IDW'01(2001), pp. 1395–1398.
FIG. 11 is a circuit diagram for describing the configuration of a current-programmed pixel circuit according to a conventional technique.
Referring to FIG. 11, a current-programmed pixel circuit of a conventional technique includes a pixel driving circuit PDC for supplying a current corresponding to instructed display luminance to an organic light emitting diode OLED provided as a light emitting element. Pixel driving circuit PDC has n-type (n-channel) TFT elements T1 and T4, p-type (p-channel) TFT elements T2 and T3, and a voltage holding capacitor Ca.
Although the details are not shown, in the whole organic EL display, pixel circuits shown in FIG. 11 are arranged in a matrix. Each pixel is associated with one scan line SL and one data line DL. Scan line SL is activated to the high level (hereinafter, also written as “H level”) in correspondence with a scan period of a corresponding pixel circuit and is inactivated to the low level (hereinafter, also written as “L level”) in the other period. To data line DL, a data current Idat corresponding to display luminance of the pixel circuit to be scanned is passed.
N-type TFT element T1 is electrically coupled between corresponding data line DL and a node Na and its gate is coupled to corresponding scan line SL. p-type TFT elements T2 and T3 are connected in series between a power source voltage Vdd and organic light emitting diode OLED. N-type TFT element T4 is electrically coupled between a connection node of p-type TFT elements T2 and T3 and node Na. The gate of p-type TFT element T2 is connected to node Na and each of the gates of p-type TFT element T3 and n-type TFT element T4 is coupled to corresponding scan line SL. The voltage of node Na, that is, a gate-source voltage (hereinafter, also simply referred to as “gate voltage”) of p-type TFT element T2 is held by voltage holding capacitor Ca connected between node Na and power source voltage Vdd.
Organic light emitting diode OLED is connected between p-type TFT element T3 and a common electrode. FIG. 11 shows a “cathode common configuration” in which the cathode of organic light emitting diode OLED is connected to the common electrode. To the common electrode, a predetermined voltage Vss is supplied. As predetermined voltage Vss, a ground voltage or a negative voltage is used.
The configuration of a current supply circuit for generating data current Idat corresponding to display luminance will now be described.
FIG. 12 is a circuit diagram showing the configuration of a current supply circuit according to a conventional technique for supplying data current Idat to a current-programmed pixel circuit.
Referring to FIG. 12, the current supply circuit according to a conventional technique has n-type TFT elements T5 to T8 and a voltage holding capacitor Cb. N-type TFT elements T5 and T6 are connected in series between data line DL and predetermined voltage Vss. N-type TFT element T7 is electrically coupled between a node to which data voltage Vdat corresponding to instructed display luminance is transmitted and a node Nm. N-type TFT element T8 is electrically coupled between a node Nb and node Nm. Node Nm corresponds to a connection node of n-type TFT elements T5 and T6.
Voltage holding capacitor Cb is connected between node Nb and predetermined voltage Vss. The gate of n-type TFT element T6 is connected to node Nb. To the gate of n-type TFT element T5, a control signal Sscn is inputted. To the gate of each of n-type TFT elements T7 and T8, a control signal Sadj is inputted.
The operation of the current supply circuit of the conventional technique will now be described.
First, in an operation mode in which control signal Sscn is set to the L level and control signal Sadj is set to the H level, n-type TFT element T5 is turned off and n-type TFT elements T7 and T8 are turned on. By the operation, a current according to data voltage Vdat is passed to n-type TFT element T6 and the gate voltage of n-type TFT element T6 for passing such a current is held at node Nb by voltage holding capacitor Cb. In such a manner, data voltage Vdat is received by the current supply circuit, the gate voltage of n-type TFT element T6 is set to the level for supplying data current Idat according to data voltage Vdat and held at node Nb.
After that, in an operation mode in which control signal Sadj is set to the L level and control signal Sscn is set to the H level, n-type TFT element T5 is turned on and n-type TFT elements T7 and T8 are turned off. By the operation, n-type TFT element T6 is electrically connected between data line DL and predetermined voltage Vss in a state where the gate voltage is held at a level for supplying data current Idat corresponding to received data voltage Vdat.
Referring again to FIG. 11, in response to activation (to the H level) of the corresponding scan line, in pixel driving circuit PDC, n-type TFT elements T1 and T4 are turned on and n-type TFT element T3 is turned off. Consequently, a current path of power source voltage Vdd, p-type TFT element T2, n-type TFT element T4, n-type TFT element T1, data line DL, n-type TFT elements T5 and T6 (FIG. 12), and predetermined voltage Vss is formed. To the current path, data current Idat corresponding to data voltage Vdat, which is according to the gate voltage of n-type TFT element T6 is passed.
At this time, in the pixel circuit, the drain and gate of p-type TFT element T2 are electrically connected to each other via n-type TFT element T4, so that the gate voltage at the time when data current Idat passes through p-type TFT element T2 is held at node Na by voltage holding capacitor Ca. As described above, in the activation period of scan line SL, data current Idat according to display luminance is programmed by pixel driving circuit PDC.
After that, when an object to be scanned is changed and scan line SL is inactivated to the L level, n-type TFT elements T1 and T4 are turned off and p-type TFT element T3 is turned on. Consequently, a current path of power source voltage Vdd, p-type TFT element T2, p-type TFT element T3, organic light emitting diode OLED, and common electrode (predetermined voltage Vss) is formed, and data current Idat programmed in the activation period of scan line SL can be continuously supplied to organic light emitting diode OLED also in the inactive period of scan line SL.
As described above, in the current-programmed pixel circuit, current supplied to the current-driven light emitting device (that is, OLED) is set on the basis of not a program of data voltage Vdat indicative of display luminance but a program of data current Idat obtained by converting data voltage Vdat. Therefore, even if a difference occurs in transistor characteristics of TFT elements of pixel circuits, non-uniformity of display luminance characteristic between pixels can be suppressed. In other words, at least between pixels sharing the current supply circuit shown in FIG. 12, uniformity of display luminance characteristic between the pixels can be expected.
However, the current supply circuit shown in FIG. 12 corresponding to the current-programmed pixel circuit has to be provided for each data line DL. Consequently, whether display luminance characteristics of pixels become uniform or not depend on whether the conversion characteristic from data voltage Vdat to data current Idat is uniform among a plurality of current supply circuits provided in a whole organic EL display.
Concretely, in the current supply circuit shown in FIG. 12, when the transistor characteristics (particularly, threshold voltage or mobility) of n-type TFT element T6 for driving data current Idat vary and uniform data current Idat cannot be generated by the current supply circuits in correspondence with data voltage Vdat at the same level, uniformity of the display luminance characteristics among pixels cannot be maintained.
In the current supply circuit according to the conventional technique shown in FIG. 12, at a timing when data line DL and the current supply circuit are connected to each other in response to activation (to the H level) of control signal Sscn, the drain voltage of n-type TFT element T6 changes discontinuously. One of problems is that data current Idat fluctuates transiently.